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I've been trying to find information on choosing an NPN transistor but everything I've found seems to apply to saturated transistors only. I'm basically trying to amplify the current given from a 3mm audio headphone jack to a useful level. Knowing the voltage and current required for the load at the collector, plus the voltage and current supplied to the base how can I determine from a spec sheet if a transistor is suitable?

Also, am I correct in thinking that the voltage supplied by the audio source will need to be at least 0.6V?

EDIT: For the sake of an example let's say the load is a bunch of LED's totaling 9V and 800mA. Obviously with an appropriate power source. I'm still researching the current and voltage of the audio signal but for this example let's say it's 0.3V and 5mA.

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  • \$\begingroup\$ Good, just current amplification. (so an emitter follower I assume.) What is your load? That will help pick a transistor. (For ~50-100mA I might try a 2N4401, above that a TIP31C... but that's just what's in my parts box.) Oh, and no you don't need 0.6 v of input. You'll bias the transistor on and capacitivly couple the input. (Class A.. wastes power, but it's easy.) \$\endgroup\$ – George Herold Sep 11 '14 at 12:33
  • \$\begingroup\$ Are you really trying to amplify the current given from a 3mm audio headphone jack or do you mean voltage or power (voltage and current)? What do you call a 'useful level'? Are you trying to build a full audio amplifier, say 10 Watts into an 8 ohm speaker or just something to amplify a few mV up to a couple of volts at a few mW. Its much easier to answer your problem when we have some numbers to work with. \$\endgroup\$ – JIm Dearden Sep 11 '14 at 13:08
  • \$\begingroup\$ @JImDearden: the load is a set of LED's totalling approximately 9V and 800mA (for the sake of example). \$\endgroup\$ – Jimbroze Sep 11 '14 at 15:54
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Simple amplification of audio is the most trivial demand for a BJT and virtually any transistor (including germanium) will do the job you want. Maybe look for BJTs that are referenced as good audio amplifiers but, given that the marketing boys would see some folk being put off buying a particular transistor because it is pigeon-holed by the label.

The level of audio does not need to be greater than 0.6 volts because the right BJT amplifier will set its bias point to accommodate all the + and - swings of the audio signal it wishes to amplify.

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Normally, to amplify an audio signal, you should know what is the excursion signal you want to achieve over the load. A good method is to use the load line.
This will let you know the maximum collector current to develop and the maximum voltage that must support the collector-emitter transistor. These are the basic parameters to select the transistor.

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